Natural Rubber is a polymer of isoprene (2-methyl-1,3-butadiene), and is one of the world's best known natural polymers. Plant-derived natural rubber, is predominately comprised of cis-1,4 polyisoprene which forms a milky suspension or dispersion in water (latex), and is found in the sap of a variety of plant species such as guayule (Parthenium argentatum), gopher plant (Euphorbia lathyris), mariola (Parthenium incanum), rabbitbrush (Chrysothamnus nauseosus), milkweeds (Asclepias syriaca, speciosa, subulata, et al), goldenrods (Solidago altissima, graminifolia, rigida, et al), pale Indian plantain (Cacalia atripilcifolia), Russian dandelion (Taraxacum Kok-Saghyz), mountain mint (pycnanthemum incanum), American germander (Teucreum canadense) and tall bell flower (Campanula Americana), amongst others.
Some plant species, such as guayule, thrive in arid and semi-arid regions such as those present in the southwestern deserts of the United States. This potentially provides an opportunity to greatly expand domestic production of natural rubber using land that would otherwise lie dormant. Natural rubber can be extracted from plants, including guayule, in the form of a latex using waterborne processes such as those detailed in U.S. Pat. No. 7,923,039 (herein the entire contents of which are incorporated by reference).
In general, the rubber latex that is derived from plants contains a few percent of other materials (proteins, fatty acids, resins and inorganic materials). In order to meet certain standards (such as those developed and administered by ASTM International), as well as market requirements and consumer preferences, it is desirable to convert the plant-derived latex into a block rubber product that is essentially free of resin and other contaminants. The conventional processes used for making Hevea block rubber, such as coagulation with dilute acids such as formic and acetic acid, tend not to completely coagulate the latex. In addition, the rubber tends to thermally and oxidatively degrade rapidly during the drying processes forming chemical groups which can create color centers in the polymer. Natural rubber that has been extracted and processed into film or block rubber also tends to degrade during storage. If present in sufficient concentration, these color centers can substantially discolor the rubber. Some end-user applications, such as those requiring use in medical, and health related applications, require a natural rubber product to meet certain chemical and physical parameters. This includes having a visual appearance requiring the material to be essentially free of color centers. In some applications, it is desirable to extract natural rubber from rubber latex that contains some proportion of antioxidant or other stabilizer product. More preferably, the antioxidant or other stabilizer should not itself substantially discolor the natural rubber product to any significant degree. Furthermore, as latex generally contains a high percentage of water, using processes designed to produce block and film rubber tends to take extended process times, or require inefficient use of energy to produce a dried product. Furthermore, direct conversion of latex to a film or block rubber product inherently produces a material with undesirable concentrations of resin and other contaminants. In certain applications where the guayule resin is considered detrimental to user-performance, it is often desirable to remove substantially all of the resins, or some of the resins. Therefore it is often desirable to remove substantially all the acetone extractable components in the rubber, other than any added stabilizer, including, without limitation, antioxidants.